1. Field of the Invention
The present invention relates to a small-sized zoom lens which provides high variable power for use in imaging devices (e.g., video cameras or digital still cameras); and an imaging device incorporating such a zoom lens.
2. Description of the Related Art
Small-sized and high-variable-power zoom lenses for use in an imaging device (e.g., a video camera or a digital still camera) are plagued with five modes of aberration, i.e., spherical aberration, coma, astigmatism, field curvature, and distortion, as well as chromatic aberration. Conventionally, various methods have been adopted to compensate for such aberrations.
In recent years, there has been an increasing desire to miniaturize various image processing devices. This has led to a need to shorten the lens length of such zoom lenses.
Such miniaturization requires not only compensation for the aforementioned five modes of aberration and chromatic aberration, but also a configuration in which incident light on the image plane becomes parallel to the optical axis of the system as much as possible (that is, the image side of the lens needs to be "telecentric"). This has presented a major design constraint to those who wish to construct a zoom lens with a short lens length.
Now, the structure of a conventional zoom lens will be described in detail.
For example, Japanese Laid-Open Publication No. 5-297275 discloses a zoom lens of a four-array rear focus type (hereinafter referred to as "Conventional Example 1"), as shown in FIG. 13. This zoom lens includes: a first lens array 101 which has positive refractive power and which is in a fixed position relative to an image plane 106; a second lens array 102 which has negative refractive power and which provides variable power by moving along the optical axis direction; a third lens array 103 which has positive refractive power and which is in a fixed position relative to the image plane 106; and a fourth lens array 104 which has positive refractive power and which is capable of moving along the optical axis direction so as to maintain the image plane 106, which moves corresponding to the movement of the second lens array 102 and the object to be imaged (hereinafter referred to as an "imaging object"), at a predetermined distance from a reference plane. The lens arrays 101 to 104 are arranged in the above order so that the first lens array 101 lies adjacent to the object to be imaged. Reference numeral 105 denotes a flat plate which is equivalent to a low-pass filter, an infrared cut filter, and/or a cover glass of an imaging device.
In accordance with the structure of Comparative Example 1, the first lens array 101, which is fixed relative to the image plane 106, provides an image formation function. The second lens array 102, which is capable of moving along the optical axis direction, provides a variable power function, i.e., ability to vary the focal length of the entire system. The third lens array 103, which is fixed relative to the image plane 106, provides a converging function. The fourth lens array 104, which is capable of moving along the optical axis direction, provides a focusing function, i.e., the ability to minimize the variation in the image-forming position responsive to any movement of the second lens array 102 and any movement of the object to be imaged.
However, with the structure of Comparative Example 1, it has been very difficult to realize a high-variable-power zoom lens of a sufficiently small size.
On the other hand, Japanese Laid-Open Publication No. 9-269452 describes a zoom lens (hereinafter referred to as "Conventional Example 2") which additionally includes a fifth lens array having negative refractive power in a four-array rear focus type zoom lens similar to that of Conventional Example 1, in an attempt to shorten the lens length of the zoom lens.
However, the zoom lenses of Comparative Examples 1 and 2 each have a problem in that, in order to achieve a high zooming ratio on the order of .times.10 with such zoom lenses, the exit pupil will inevitably be located near the image plane. As a result, it is difficult to achieve telecentricity on the image side. These zoom lenses also have a problem in that their lens length cannot be sufficiently shortened, which makes it difficult to miniaturize such zoom lenses.